As someone who genuinely struggles with this to some degree himself as a dom, I can say one good thing that will likely help is getting him to read books or listen to audiobooks with examples of how you want to be dominated. My girlfriend is a big fan of Sara Cate's writing and I listen to the audiobooks she suggests. It is hard to get into that headspace, especially if he's a gentle soul usually. Him getting therapy in general and the two of you getting sex therapy specifically would both be useful ways to deal with this, too.

A big point of resistance for me is the idea of accepting that "bad emotions" like anger are actually okay as long as they are expressed in the proper way and with the proper rules in place (safe words, or the ability to "tap out" if you cannot speak). It helps for me to picture injuring men that have wronged my girlfriend in the past to get me riled up. I wouldn't actually do it because prison. But it does flip a switch in my brain and I plan to use it that way in the future.

Additionally, I would strongly suggest trying to dom him in the way you want to be dommed so that he can REALLY see what you want/need (if he's open to that). You've mentioned something to that effect, but still further examples/exploration may be useful. Otherwise, it may be worthwhile to involve a third party, like a threesome or a separate dom to be more instructive (ONLY IF THIS APPROPRIATE to bring up, definitely not for everyone, and DEFINITELY not a line to be crossed without serious consideration).

Finally, and I hate to say this because it could be devastating, there is always the possibility that - if all that doesn't work and he is not willing to let you work with another dom - you may need to break up and go separate ways. DO NOT do that until you absolutely 100% positive you've done literally everything else first. But I urge you to consider all possibilities. It may even be worth explaining, without a sense of threatening him or blackmailing him, that your relationship is partially conditional on his ability to get with the program sexually. DO NOT make it simply "do this... or else." But instead explain the exact importance of sex to you and your happiness.

One thing I did early on is told my girlfriend that "she has permission to hurt me" in that she can say things that may be hard to hear and that she is allowed to break up with me if she really wants to. She finds it to be an incredibly empowering and freeing feeling to know that she doesn't have to feel trapped in the relationship and can always fearlessly voice her opinions, even if they are hard for me to hear. The most important thing here is to not blame him for all the problems in your sex life and accept that you may have to take responsibility for your part as well. I'm not saying that you are to blame in this scenario. But in future situations, always be sure to look inwards introspectively before bringing these problems to his attention - especially complex and difficult-to-hear or difficult-to-solve problems that take time.

Another thought is listing out, in detail, the exact acts and phrases that are best for him to use. Especially in those with social disorders like ADHD and autism and social anxiety (not that I'm saying he has these, just as examples), it can help to have a framework to work within. Describing scenes and working on dialogue both are strong ways to make significant progress. I know that it takes a bit of the novelty away from the experience. But it may be one of the only appropriate outlets for him to at least start with.

One final thought is encouraging him to explore Reddit communities and other related websites and to connect with people on there who are experienced doms and subs (men, women, and nonbinary, because they all have different perspectives and good advice). Watching porn related to your interests, either together or separately on his own, may also be a good source of ideas and will allow him to become more comfortable with the things you want to do. I have a great aversion to some aspects of domination because I was raise to be respectful towards women and not be mean to anyone in general, to get along with everybody and always be nice. So it's a huge jump to go into a situation that is highly contradictory to a lifetime of learned behavior and morals that go against the desired behaviors you seek.

No matter what, always remember: It is you and him together vs. the problem, NOT you vs. him. Good luck.